1. Field of the Invention
This invention relates to a radiation image storage panel for recording and reproducing a radiation image using a stimulable phosphor which stores radiation energy and emits light upon stimulation thereof, and more particularly to a radiation image storage panel exhibiting improved physical properties in which a stimulable phosphor is dispersed in a binder comprising linear polyester resin or liner polyester resin crosslinked with a crosslinking agent to form a fluorescent layer.
2. Description of the Prior Art
A novel method for obtaining a radiation image is disclosed, for example, in U.S. Pat. Nos. 3,859,527, 4,236,078 and 4,258,264. In the method of the patents, there is used a radiation image storage panel comprising a stimulable phosphor which emits light when stimulated by an electromagnetic wave selected from among visible light and infrared rays after exposure to a radiation. (The term "radiation" as used herein means electromagnetic wave or corpuscular radiation such as X-rays, .gamma.-rays, .beta.-rays, .gamma.-rays, high energy neutron rays, cathode rays, vacuum ultraviolet rays, ultraviolet rays, or the like.) The method comprises the steps of (i) causing the stimulable phosphor of the panel to absorb a radiation passing through an object, (ii) scanning the panel with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the panel as light emission, and (iii) electrically converting the emitted light into an image.
The radiation image storage panel employed in the above-mentioned method for recording and reproducing a radiation image comprises a substrate and a fluorescent layer provided thereon. The fluorescent layer comprises a binder and a stimulable phosphor dispersed therein. Usually, a protective layer for physically and chemically protecting the fluorescent layer is provided on the naked surface of the fluorescent layer. Further, a primer layer is sometimes provided between the fluorescent layer and the substrate to closely bond the fluorescent layer to the substrate.
The radiation image storage panel having the above-mentioned structure is generally prepared by the following manufacturing process.
A stimulable phosphor is mixed with a binder in a suitable mixing ratio using a suitable solvent to prepare a dispersion in which the stimulable phosphor is dispersed in a solution of the binder. Then, the dispersion thus obtained is uniformly applied to a substrate by means of a doctor blade, a roll coater, a knife coater, or the like and dried to form a fluorescent layer. In the preparation of the radiation image storage panel having a primer layer between the substrate and the fluorescent layer, the primer layer is provided on the substrate beforehand, and then the dispersion is applied to the primer layer and dried to form the fluorescent layer. After the formation of the fluorescent layer, a protective layer for protecting the fluorescent layer is generally provided on the fluorescent layer. Unless otherwise indicated, the term "substrate" as used herein also means a substrate on one surface of which an intermediate layer such as the primer layer mentioned above or the light-reflecting layer described hereinbelow which is located between the substrate and the fluorescent layer in the panel to be obtained is provided beforehand.
The radiation image storage panel is incorporated in a radiation image recording device in the recording step of the above-mentioned method for recording and reproducing a radiation image. The panel is also incorporated in a radiation image read out device in the reproduction step of the method. Therefore, it is desired for the radiation image storage panel to have a high flexing resistance and a high adhesion of fluorescent layer to substrate. That is, the fluorescent layer of the radiation image storage panel should not readily craze and peel off from the substrate when the panel is bended.
The radiation image storage panel is exposed to stimulating rays in the reproduction step. Further, after the reproduction of radiation image, the panel is exposed to a light to remove the radiation energy remaining in the panel as disclosed in Japanese Unexamined Patent Publication No. 11,392/1981. (The light for removing the radiation energy remaining in the panel is hereinafter referred to as "erasing rays".) Since the radiation image storage panel is repeatedly used, the panel is repeatedly exposed to stimulating rays and erasing rays over a long period of time. Therefore, it is desired for the radiation image storage panel to have a high resistance to stimulating rays and erasing rays. That is, the sensitivity and the flexing resistance of the radiation image storage panel should not be readily lowered when the panel is exposed to stimulating rays or erasing rays for a long period of time. Accordingly, a material which is difficult to be yellowed by stimulating rays or erasing rays and therefore, hardly absorbs stimulating rays and/or the light emitted by the stimulable phosphor and hardly reduces the sensitivity of the panel should be used as the binder of the fluorescent layer of the panel. Further, a material which is difficult to be further cured by stimulating rays or erasing rays and therefore, hardly reduces the flexing resistance of the panel should be used as the binder.
The above-mentioned structure of the radiation image storage panel is almost the same as that of a radiographic intensifying screen except that a stimulable phosphor is used in place of a radioluminescent phosphor. In the conventional radiographic intensifying screen, cellulose derivatives such as nitrocellulose and cellulose acetate have been practically used as the binder of the fluorescent layer of the screen. Accordingly, it is intended to use these cellulose derivatives as the binder of the fluorescent layer of the radiation image storage panel.
In a dispersion for forming a fluorescent layer of a radiation image storage panel in which cellulose derivative is employed as the binder of the fluorescent layer, a stimulable phosphor is dispersed uniformly. However, the radiation image storage panel prepared by using the dispersion exhibits low performance in flexing resistance and adhesive force of fluorescent layer to substrate. Accordingly, the fluorescent layer of the panel readily crazes and peels off from the substrate when the panel is bended. Further, the radiation image storage panel in which cellulose derivative is employed as the binder of the fluorescent layer exhibits low performance in resistance to stimulating rays and erasing rays. That is, the cellulose derivative employed as the binder of the fluorescent layer of the panel is easily yellowed by stimulating rays or erasing rays, and therefore, the sensitivity of the panel is remarkably lowered when the panel is exposed to stimulating rays or erasing rays for a long period of time. Further, the cellulose derivative is further cured by stimulating rays or erasing rays, and therefore, the flexing resistance of the panel is also lowered by exposure to stimulating rays or erasing rays.
As described above, the radiation image storage panel in which cellulose derivative is used as the binder of the fluorescent layer exhibits low performance in flexing resistance, adhesion of fluorescent layer to substrate, and resistance to stimulating rays and erasing rays. Accordingly, a radiation image storage panel exhibiting higher performance in flexing resistance, adhesion of fluorescent layer to substrate, and resistance to stimulating rays and erasing rays than the radiation image storage panel in which cellulose derivative is used as the binder of the fluorescent layer is desired.